1. Field of the Invention
The present invention relates to radio communications systems and radio communications methods in an environment in which multiple radio stations share the same frequency.
2. Description of the Related Art
In current radio communications, a dedicated frequency band is often allocated for each of radio communications systems in order to avoid mutual interference. However, in recent years, methods of utilizing the same frequency band in multiple communications systems are being considered in order to effectively utilize limited frequencies in radio communications. In order to share the same frequency band among multiple radio stations or multiple radio communications systems, it is necessary to mutually recognize a frequency usage condition and to perform transmission control so as not to cause interference. Transmission control schemes are being considered which may roughly be categorized into two types: distributed control, in which individual radio stations observe the frequency usage condition in the surroundings and in which the individual radio stations determine whether a transmission can be conducted; and centralized control, in which a control station which centrally manages the frequency usage condition of multiple radio stations determines, for each of the radio stations, whether the transmission can be conducted.
Conventionally, there has been a technique which efficiently reuses the same frequency among different cells. In a normal cellular system, a technique called an inter-cell frequency reuse is being used. The inter-cell frequency reuse utilizes a different frequency between neighboring cells to avoid inter-cell interference. A technique called interference coordination, which is being proposed in “3GPP.R1-060670” (Non-patent document 1) provides a frequency to be allocated to a radio station located at a cell edge and sets the cell-edge frequency to differ between neighboring cells to avoid inter-cell interference. Moreover, JP2007-258844A (Non-patent document 1) proposes a technique in which radio stations are grouped based on a report of CQI (channel quality indicator) information from terminals and a resource block of different transmission power is allocated to each of the respective groups to reduce effect of interference received by the cell-edge base station. Patent document 1 also proposes a technique in which a base station measures an interference power level with a neighboring cell and varies, between cells with large interference, power levels allocated to the same resource block to prevent an occurrence of interference caused by using the same resource block near a cell border.
JP2008-278273A (Patent document 2) discloses a technique in which resource allocation information is shared among radio stations within the same system, and an interference region resource in which interference may occur is not allocated. These techniques are centralized control-type radio communications system techniques in which information on frequency utilization conditions is shared among surrounding radio stations in advance and a resource to be used by each of the radio stations is determined such that a control station which collected the information does not cause interference.
As a technique of sharing the same frequency in a distributed control-type radio communications system, a carrier sense multiple access scheme with a collision avoidance function, called a CSMA/CA (carrier sense multiple access with collision avoidance) scheme is known. This is described in Non-patent document 2, for example. The CSMA/CA scheme, which is used in this type of wireless LAN system, measure a received level, before transmitting data, to determine whether the data can be transmitted. Using the CSMA/CA scheme as an access scheme in the same frequency bandwidth sharing environment by multiple different radio communications systems makes it possible for each of the radio stations to conduct transmission only when a signal is not detected to avoid interference, since each of the radio stations determines whether the transmission is possible based on information on a radio communications environment that is collected by the corresponding radio station on its own.
JA2006-222665A (Patent document 3) discloses a technique in which a feature amount of a periodic steadiness of a signal is calculated to detect the presence of the signal and each radio station conducts transmission only when the signal is not detected by the own station to avoid interference. Using this technique makes it possible to detect the presence of a signal used in the surroundings even when the same frequency band is shared among different radio communications systems and makes it possible for the radio station to conduct transmission when it is determined that interference would not occur.    Patent document 1: JP2007-258844A    Patent document 2: JP2008-278273A    Patent document 3: JP2006-222665A    Patent document 4: JP2008-061214A    Non-patent document 1: 3GPP.R1-060670    Non-patent document 2: IEEE802.11 Part 11 Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications, ANSI/IEEE Std 802.11, 1999 Edition    Non-patent document 3: Matsuda, Takiguchi, Ariki, “Voice activity detection with 3rd order cumulant voice feature”, IEICE (Japan), technical report of IEICE, vol. 106, No. 263 (20060919), vol. 37-42    Non-patent document 4: IEEE 802.22 Working Group of the LAN MAN Standards Committee, “IEEE P802.22/D0.1 Draft Standard for Wireless Regional Area Networks Part 22: Cognitive Wireless RAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Policies and procedures for operation in the TV bands”, The Institute of Electrical and Electronics Engineers, May 2006.
When the same frequency band is shared among multiple different radio communications systems, it is not necessarily the case that information may be exchanged among the radio stations. Therefore, controlling a radio resource based on centralized control leads to a concern for a possible significant degradation in characteristics. More specifically, the related-art technique in Non-patent document 1 discloses distinguishing a user using a spread code and sharing the same frequency. However, it is not necessarily the case that information such as the spread code, etc., may be utilized among the radio stations utilizing the different radio communications systems. Moreover, the technique to suppress interference through interference coordination on cell-edge radio stations fails to make it possible to perform a fine cell design in an anticipated frequency shared environment, possibly leading to an occurrence of a significant degradation in characteristics in a geographical region due to the interference. Moreover, the related-art technique in Non-patent document 1 discloses a base station grouping mobile stations based on a report of CQI information from the mobile stations, and allocating a resource for each of the groups. However, sharing of the CQI information using a radio signal is not possible among radio stations utilizing different radio communications systems, leading to a possibility that such grouping and a proper resource allocation cannot be performed. Similarly, also the technique in Patent document 2 does not provide for sharing resource allocation information in an anticipated environment, leading to a possibility that a resource which makes it possible for the interference to be avoided cannot be specified and the characteristic may be significantly degraded. In this way, with a centralized control-type radio communications system, there is a problem that complexity is high in the sense that many sets of information are required in a condition such that information sharing among systems is difficult (e.g., the same frequency band sharing environment among the different radio communications systems).
On the other hand, a distributed control-type radio communications system which uses the related-art CSMA/CA scheme and the technique in Patent document 3 requires, in a situation in which information is not shared sufficiently, that whether transmission by an own station can be conducted be autonomously determined so as not to cause interference with other stations. Consequently, there is a problem that, even when there is a communication being actually conducted in the surroundings, it may be determined by a radio station on its own, as a result of monitoring the radio communications environment of the other radio stations, that a radio resource actually being used in the above-mentioned communication is available, and the radio resource is used by the own radio station, frequently causing interference. Conversely, a problem may arise such that, even when a radio resource is not being used, it is erroneously determined that the radio resource is being used and the radio station refrains from conducting transmission, causing a decrease in frequency utilization efficiency. In this way, it is considered that, compared with the centralized control-type radio communications system, the distributed control-type radio communications system makes it possible to avoid interference even in a situation in which information sharing is less sufficient, has less complex processing, but has inferior interference avoidance effect and frequency utilization efficiency.